Earlier quoted context omitted.
> You can have third-party Android app stores. This is only half-true. Google allows users to install third-party stores. Because only handful of geeks do that. But if your competing app store gains enough adoption and you make deal with device vendors to preinstall it, the negotiations will mysteriously break down and the other party will grow quiet... Reportedly, because their (NDA-covered) contract with Google doe…
How come this never happened to Amazon and Samsung app stores?
There is a huge difference between treatment of different parties, even if the their contracts are same (and I doubt, that Samsung has the same contract with Google as everyone else). For example, do you know, that some Android devices aren't allowed to use a different launcher, other than supplied by Google? Or that Google Play Terms of Service prohibit publishing there alternative app stores? There are many such draconian restrictions in all Google's contracts. They are ignored until it is convenient for Google to enforce them.
Back to your question, — neither Samsung nor Amazon are Google's competitors. Their app stores are garbage. Samsung was trying to jump off Android for years. And Amazon devices don't come with Google's software, so Google has no leverage against them (except YouTube).